The material having a diameter of 1 μm or smaller which is finer than carbon fibers is generally called carbon nanotubes and distinguished from the carbon fibers. However, there is no particularly definite boundary therebetween. By a narrow definition, the material whose carbon faces with hexagon meshes are substantially parallel to an axis is called a carbon nanotube, and even a variant of the carbon nanotube, around which amorphous carbon exists, is included in the carbon nanotube (Note that the narrow definition is applied to the carbon nanotube according to the present invention.).
Usually, the narrowly-defined carbon nanotubes are further classified into two types: carbon nanotubes having a structure with a single hexagon mesh tube (graphene sheet) are called single wall nanotubes (hereinafter, simply referred to as “SWNT” in some cases); and on the other hand, the carbon nanotubes made of multilayer graphene sheets are called multi-wall nanotubes (hereinafter, simply referred to as “MWNT” in some cases). The carbon nanotubes of these types have an extremely finer diameter than that of the carbon fibers, a high Young's modulus, and high electrical conductivity, thereby attracting attention as a new industrial material.
Thus, the carbon nanotube is a new material whose structural element is only carbon, and is mechanically extremely strong enough to exceed a Young's modulus of 1 TPa. In addition, electrons flowing through the carbon nanotube easily undergo ballistic transport, so that it is possible to flow a large quantity of current. Further, the carbon nanotube has a high aspect ratio, so that its application to a field electron emitting source is underway, and a light emitting element and display having a high brightness is under development. Furthermore, some single wall carbon nanotubes exhibit semiconductor characteristics, and are applied to the experimental manufacture of a diode and a transistor. Therefore, its application is especially desired in a field of functional materials and in a field of an electronic industry.
Conventionally, it has been known that fullerenes and carbon nanotubes can be manufactured by methods including resistance heating, plasma discharge such as arc discharge with a carbon rod as a raw material, laser ablation, and chemical vapor deposition (CVD) using acetylene gas. However, a mechanism of generating carbon nanotubes with those methods is controversial in various respects, and a detailed growth mechanism is not disclosed even now.
With regard to the manufacture of a carbon nanotube, various methods and improvements have been studied for the purpose of synthesis in a large quantity. The resistance heating which was devised in the early stage is a method of heating and vaporizing graphite by bringing the forward ends of two pieces of graphite in contact with each other in a rare gas, and then applying a current of several tens to several hundreds of amperes. However, with this method, it is extremely difficult to obtain a few grams of specimen, so that the method is hardly used now.
The arc discharge is a method of synthesizing fullerenes and carbon nanotubes by generating arc discharge in a rare gas such as He and Ar while using graphite rods as an anode and a cathode. The forward end portion of the anode reaches a high temperature of 4000° C. or more by arc plasma generated by the arc discharge, then the forward end portion of the anode is vaporized, and a large quantity of carbon radicals and neutral particles are generated. The carbon radicals and neutral particles repeat collision in the plasma, further generate carbon radicals and ions, and become soot containing fullerenes and carbon nanotubes to be deposited around the anode and cathodes and on the inner wall of an apparatus. When the anode includes an Ni compound, a ferrous compound, or a rare earth compound, which acts as catalyst, single wall carbon nanotubes are synthesized efficiently.
The laser ablation is a method of irradiating a pulse YAG laser beam on a graphite target, generating high density plasma on the surface of the graphite target, and generating fullerenes and carbon nanotubes. The characteristic of the method is that a carbon nanotube having a relatively high purity can be obtained even at a growth temperature of more than 1000° C.
A technique for higher purity synthesis of the SWNT for the purpose of increasing the purity in the laser ablation is reported in A. Thess et. al, “Nature”, Vol. 273, p. 483-487. However, the laser ablation supplies only a small quantity of carbon nanotubes, and the efficiency is low, leading to higher costs of carbon nanotubes. In addition, the purity remains about 70 to 90%, and is not sufficiently high.
The chemical vapor deposition is a method of generating carbon nanotubes by a chemical decomposition reaction of the raw material gas, using an acetylene gas, a methane gas, or the like containing carbon as a raw material. The chemical vapor deposition depends on a chemical reaction occurring in a thermal decomposition process of the methane gas and the like serving as the raw material, thereby enabling the manufacture of a carbon nanotube having a high purity.
However, in the chemical vapor deposition, the growth rate of the carbon nanotube is extremely low, so that the efficiency is low and the industrial application is difficult. In addition, the structure of the manufactured carbon nanotube has more defects and is incomplete compared with that synthesized in the arc discharge or the laser ablation.
The use of a vertical furnace enables continuous growth, thereby realizing a growth apparatus having a high production capability. However, in that case, the purity of the obtained carbon nanotube remains low.
Electrons, ions of carbon, radicals, and neutral particles in the arc plasma generated by the arc discharge repeat recollision, thereby generating complex chemical reactions, so that it is difficult to stably control the density and the kinetic energy of the carbon ions. Thus, a large quantity of amorphous carbon particles and graphite particles are generated simultaneously along with the fullerenes and the carbon nanotubes, all of which exist in a mixed state as soot.
Therefore, when the fullerenes and the carbon nanotubes are to be used for the industrial application, it is necessary to purify and separate only the fullerenes and the carbon nanotubes from the soot. In particular, the carbon nanotubes does not dissolve in a solvent, so that the purification thereof is conducted by combining centrifugation, oxidation, filtering, and the like. However, physical properties and chemical properties of the carbon nanotubes, and those of the amorphous carbon particles and the graphite particles, which are major impurity, are approximately the same, thereby making it difficult to remove the impurity completely. Thus, high purity carbon nanotubes are obtained by repeating purification. It is also known that, in the purification process, alkali metal may remain due to the influence of a surface active agent used as a dispersing agent, and the influence of the mechanical damage is extensive as well in the purification process, thereby causing a large quantity of defects in the carbon nanotubes.
To solve this problem, in the synthesis stage of the carbon nanotubes, a synthesis technique is desired for high purity carbon nanotubes including as less impurities as possible, that is, such carbon nanotubes as to include no amorphous carbon particles nor graphite particles.
Therefore, an object of the present invention is to solve the problems of the above-mentioned conventional art. Specifically, the present invention has an object to provide a manufacturing apparatus and method which can efficiently synthesize a high purity carbon nanotube having an extremely low concentration of impurities such as the amorphous carbon and graphite particles on an industrial basis.